Last month, a group of General Motors Corp. executives showed Vice President Dick Cheney a pickup truck that is partly electric-powered, hoping to impress him with its environmentally friendly technology. They hooked up a popcorn popper to an outlet on the prototype vehicle and offered Mr. Cheney a batch.

But minutes earlier, the message from Chief Executive G. Richard Wagoner Jr. had been decidedly less green. At a private meeting outside Detroit, GM warned Mr. Cheney and several GOP congressmen that if fuel-economy...